The following disclosure generally relates to electrical circuits and signal processing.
Conventional amplifiers receive an input voltage and produce an output voltage according to a gain, where the gain is defined as a ratio of the output voltage to the input voltage. One type of amplifier is a chopper amplifier (sometimes referred to as a tuned amplifier).
FIG. 1 is a schematic diagram illustrating a conventional chopper amplifier 100. Chopper amplifier 100 includes a modulator 110, an operational amplifier 120, a demodulator 130, a filter 140, feedback resistors 150(1),(2), and input resistors 160(1),(2). Feedback resistors 150(1),(2) are coupled across the respective outputs and inputs of operational amplifier 120. A DC offset signal 153 is also coupled to an input of operational amplifier 120. Modulator 110 modulates an input signal 102 in accordance with a frequency of a carrier signal 104. Operational amplifier 120, amplifies the modulated input signal according to a gain defined as a ratio of feedback resistance to input resistance. Demodulator 130 returns the amplified signal to an original frequency. Lastly, filter 140 removes DC offset and DC noise, and produces output signal 190.
Conventional chopper amplifier performance suffers from many drawbacks. One problem is that chopper amplifiers are usually not able to maintain high gains. In a conventional chopper amplifier, a resistance associated with modulation switches unduly increases an input resistance, thus limiting gain. Another problem is that a DC offset applied to the operational amplifier may be amplified by the gain, causing a reduced output swing voltage that is insufficient for low voltage applications (e.g., 1.8V or 2.3V). Removal of the DC offset subsequent to an output of the operational amplifier fails to address the loss of full voltage output swing at the output of the operational amplifier. Furthermore, the amplified DC offset, if sufficiently high, can saturate the operational amplifier or subsequent circuitry. Yet another problem is that voltage-type operational amplifiers typically used in conventional chopper amplifiers diminish gain bandwidth with increase in gain, thereby requiring a low carrier frequency and a bulky output filter.